1. Field of the Invention
This invention relates to packaging of semiconductor integrated circuits and, more particularly, to a leadframe that directs the molding compound to flow evenly into the package during the encapsulation process.
2. Description of the Related Art
Semiconductor packaging generally involves incorporating completely fabricated chips, generally referred to as a die, into protective packages so that the dies are protected from environmental contaminants and handling damage. A common method for packaging integrated circuits comprises bonding each die to a leadframe and subsequently encapsulating the die and a portion of the leadframe in a molded epoxy enclosure. Generally, the leadframe has paddles that receive the die and also has lead fingers that provide easier contact points for external electrical connection to the electronic components of the die. Hence, the leadframe provides structural support for the die and allows the encapsulated die to establish connection with external structures such as printed circuit boards.
More particularly, the leadframe is typically made from a thin sheet of metal and the die paddle that is configured to receive the die is typically downwardly recessed to the rest of the frame so as to accommodate for the thickness of the die mounted to the paddle. Furthermore, the leadframe comprises a plurality of leadframe fingers that are either stamped or etched on the leadframe and extend from an edge of the die paddle to an edge of the leadframe. An inner end of each leadframe finger is wire bonded to a bonding pad on the die while an outer end of each finger is designed to form contacts with external structures so as to establish a plurality of conductive paths between the die and the external structures. The number and configuration of leadframe fingers vary depending on the particular die design.
Moreover, an inner section of each leadframe finger and the die itself are typically encapsulated in a plastic enclosure so that they are protected from damage and contaminants. During the encapsulation process, the die is initially bonded to the leadframe and the die and leadframe structure is placed in a mold cavity wherein the configuration of the cavity defines the shape and size of the resulting protective enclosure. Furthermore, a molding compound such as plastic resin is injected into the cavity so as to complete the formation of a typically rectangular shaped enclosure that protects the die and the inner sections of the leadframe fingers. Advantageously, the molding operation can be set up so that multiple dies can be encapsulated in a single process run, thereby providing a cost effective and efficient way of packaging integrated circuits.
However, one disadvantage of the standard encapsulation process is that the mold cavity configuration and positioning of the leadframe therein preclude the molding compound from flowing evenly inside the cavity. This can cause an uneven distribution of resin around the die and the leadframe resulting in a less effective protective structure. In particular, the mold cavity typically comprises a top and a bottom plate that mate to form the cavity. The leadframe is placed in the middle of the cavity between the two plates. Furthermore, the mold is typically designed so that the molding compound or resin enters the cavity from an opening or gate formed in one corner of the mold. In a bottom gated mold configuration, for instance, the molding compound enters the cavity from a bottom corner and the molding compound fills the bottom portion of the cavity more quickly than it fills the top portion of the cavity. Hence, molding compound entering from a bottom gated mold tends to conglomerate near the bottom of the cavity because its upward flow path is hindered by numerous horizontally extended leadframe fingers. Likewise, in a top gated mold configuration, the flow of the molding compound also can conglomerate in the top portion of the cavity as the downward flow path is obstructed by the horizontally extended leadframe fingers. Disadvantageously, non-uniform compound flow rate in the mold and uneven resin distribution between the top and bottom surface of the leadframe are known to cause defects such as voids, pinholes, and knitlines in the cured plastic enclosure.
To address this problem, mold design modifications and leadframe alterations have been developed in the past in an effort to achieve a more uniform resin flow inside the mold cavity. For example, U.S. Pat. No. 5,965,078 discloses a mold design used in combination with prepackaged molding compound inserts that are inserted in the cavity of the mold with the leadframe and die so as to provide a more uniform compound flow inside the cavity. However, it can be appreciated that mold modifications with prepackaged inserts are costly to implement and therefore undesirable in light of the ever increasing demand for cost reduction in semiconductor fabrication.
The prior art also discloses a leadframe having one or more encapsulate diverters wherein the flow diverters are designed to guide portions of the upper resin flow to a bottom section of the cavity. (See e.g., U.S. Pat. No. 5,926,695). However, the flow diverter as taught by prior art is applicable only for molds that are top gated wherein the resin is introduced from a top comer of the mold. Furthermore, the diverter is positioned away from the gate and therefore unable to direct the flow of the compound at the point where the resin first enters the cavity. In fact, the diverter is designed to guide the resin flow only after the resin has already reached the paddle area where air pockets that are known to cause voids and pinholes are likely to have already developed.
Moreover, the leadframe diverters as suggested by prior art are essentially unsupported flaps that can be easily damaged during the encapsulation process. In particular, it can be appreciated that an unsupported thin layer of metal bent at an angle can be deformed or broken off by the flow of the compound injected into the mold cavity. Furthermore, leadframes with the flow diverters as suggested by prior art are time consuming to manufacture as it requires additional processing steps to ensure that the diverters bend at a particular angle relative to the leadframe.
Further, the flow diverter disclosed in the patent also requires that space on the die paddle be occupied by the opening to permit the resin to flow through. It will be appreciated that given the ever increasing need for greater density devices, the space on the die paddles and on the lead frame is becoming increasingly limited. Hence, for many high density leadframe designs, forming openings simply for resin flow is impractical and inefficient.
Hence, from the foregoing, it will be appreciated that there is a need for a leadframe wherein the leadframe directs the flow of the molding compound to flow more evenly inside the mold cavity during the encapsulation process. To this end, there is particular need for an altered leadframe that is able to guide the flow of the compound as the compound enters the cavity so as to achieve a uniform resin flow rate from the outset and thereby minimize defects such as pinholes, voids and knitlines in the molded enclosure. Furthermore, it is desirable that the leadframe is effective in directing compound flow in both top and bottom gated molds. Furthermore, it is also desirable that such leadframe alterations can be implemented quickly and cost effectively.
The aforementioned needs are satisfied by the leadframe of the present invention. In one aspect, the present invention discloses a leadframe wherein the leadframe comprises a plurality of lead fingers and a die paddle adapted to receive the die and a second offset positioned adjacent an edge of the leadframe. In this aspect, the second offset is positioned so as to increase the pressure differential between the first and the second surfaces of the leadframe so as to increase the flow of compound from the first surfaces to the second surface via the spaces between the plurality of lead fingers. In this way, the flow of compound can be increased without requiring limited space on the leadframe be occupied by flow openings and the like.
In one embodiment, the offset comprises a downset comprising a downward indentation formed on a top surface of the leadframe wherein a top surface of the indentation slopes upwardly towards the paddle until it becomes level with a general plane defined by the top surface of the leadframe.
In one embodiment, the leadframe further comprises a tie bar wherein an outer end of the tie bar is generally triangular and comprises the offset. Preferably, in an encapsulation process, the leadframe is placed inside a mold cavity in a manner such that the offset is positioned adjacent a gate. Preferably, the offset is designed to receive a portion of the molding compound entering from the gate and guide the portion to flow toward the second surface of the leadframe. In particular, the offset produces a localized increase in the pressure of the compound flowing from the gate. This localized increase in pressure results in a greater pressure differential between the side of the leadframe adjacent the gate and the side opposite the gate thereby increasing the flow from the side adjacent the gate to the side opposite the gate.
Advantageously, the altered leadframe of the preferred embodiment provides an offset comprising a sloped surface strategically positioned to facilitate the flow of the molding compound as the compound enters the cavity from a gate. In particular, the altered leadframe splits the flow of the compound as the compound enters from a gate so as to ensure a more even distribution of the compound between the top and bottom of the leadframe.
From the foregoing, it will be appreciated that the aspects of the present invention provide a new, altered leadframe that is designed to direct molding compound to flow more evenly inside a mold cavity in a cost-effective manner. Moreover, the altered leadframe can be adapted to direct compound flow inside mold cavities that are either top or bottom gated. These and other objects and advantages of the present invention will become more apparent from the following description taken in conjunction with the following drawings.